This application relates to turbine blade passage cooling, and in particular relates to improvements in cooling performance of turbine blade passages over a range of buoyancy numbers.
The temperatures within gas turbines may exceed 2500 degrees Fahrenheit (1370.degree. C.), and cooling of turbine blades is very important in terms of blade longevity. Without cooling, turbine blades would rapidly deteriorate. Improved cooling for turbine blades is very desirable, and much effort has been devoted by those skilled in the blade cooling arts to devise improved geometries for the internal cavities within turbine blades in order to enhance cooling.
Gas turbine blades have historically used compressor bleed air as the cooling medium to obtain acceptable service temperatures. Cooling passages associated with this design technology are typically serpentine arrangements along the mean camber line of the blades. The camber line is the locus of points between the low pressure and high pressure sides of the airfoil. Adjacent radial passages are connected alternately at the top and bottom by one hundred and eighty degree return U-bends to form either a single continuous passage, or independent serpentine passages, with the cooling air exiting into the gas path.
Each radial passage typically cools both the high pressure and low pressure sides of the blade airfoil. The specific geometry of each radial cooling passage is designed to balance the conflicting demands for low pressure drop and high heat transfer rate. Schemes used in the art to enhance heat transfer rate within the cooling passages include raised rib turbulence promoters, also known as trip strips or turbulators, passage crossover impingement, impingement inserts, and banks or rows of pins. These schemes increase the local turbulence in the flow and thus raise the rate of heat transfer.
Cooling schemes involving high pressure and high density fluids, such as steam, have been utilized. The use of steam as a coolant for gas turbine blade cooling can provide several advantages. One advantage is superior heat transfer. For example, when comparing typical high pressure extraction steam to compressor bleed air, steam has about a 70% advantage in heat transfer coefficient in turbulent duct flow by virtue of steam's higher density and higher specific heat.
Due to the large physical size of turbines, coupled with the use of high density cooling fluids, high centrifugal buoyancy effects occur in the cooling passages of the turbine blades. With air-cooled blades, undesirable buoyancy effects are typically small, Buoyancy number (Bo)&lt;&lt;1. The buoyancy effects are greater with steam, however, and as the buoyancy factor Bo exceeds about 0.1, the undesirable effects become increasingly significant. The internal coolant passages for a steam cooled system must therefore be designed to account for Coriolis and buoyancy effects, also known as secondary flow effects, explained in greater detail below.
More specifically, at the higher densities and low flow rates of steam, the cooling steam flow within the internal blade cooling passages is prone to develop secondary flows from Coriolis and centrifugal buoyancy forces. The occurrence of these secondary flows within the cooling passages affects the predictability of heat transfer and, more critically, impairs the heat transfer because of uneven heat pickup and flow reversal.
More specifically, as a typical blade rotates about a shaft axis, one side of the airfoil is always ahead of the other, in the direction of rotation. The convex side of the airfoil is called the suction side, the concave side is called the pressure side. The suction side of the airfoil always leads the pressure side of the airfoil in rotation. Inside the airfoil, when the cooling medium is in radial outflow, the flow in the center of the passage tends to move from the region of the cavity near the suction side to the region of the cavity near the pressure side, in the plane of the cavity cross-section. This flow then returns to the suction side along the heated passage walls, thus transporting heated coolant towards the suction side. (See FIG. 3) At the suction side, the local radial flow velocities are decreased, and can even be reversed in direction, when the bulk flow is in the radial outward direction, due to the large centrifugal buoyancy effect. Because high pressure steam is more dense than air, the body forces are higher when steam is the coolant, hence the tendency of the flow to migrate, in the cavity, from the suction side to the pressure side is greater. This migration within the cavity leads to low heat transfer coefficients on the suction side in radial outflow. In radial inflow passages the Coriolis induced flow is reversed in direction from that in radial outflow passages. Since the buoyancy effect causes the hotter fluid to further accelerate in the radial inward direction, the heat transfer coefficients are increased rather than decreased.
It has thus been determined that Coriolis and buoyancy forces or effects are most significant, with respect to the local heat transfer coefficients, in radial outflow cooling passages of a serpentine cooling circuit, particularly in the region from the pitchline (halfway between the hub and the tip of the blade) to the tip of the blade.
In current designs, the heat transfer coefficients in radial cooling passages with radial outflow of fluid are severely reduced over a range of Buoyancy numbers. In order to avoid the reduced performance region, fluid velocities are increased to reduce the Buoyancy number below about 0.10. This increase in fluid velocity, in turn, increases the required number of cooling passages and increases the passage pressure drop. Furthermore, there is a large change in heat transfer coefficient for a small change in fluid flow rate, meaning that small changes in coolant flow rate will produce large changes in metal temperature. A good design should be relatively insensitive to small changes in the coolant flow rate, so this condition should be avoided to achieve a more robust design.
Therefore, it is apparent from the above that there exists a need in the art for improvements in turbine blade cooling passages that increase cooling performance over a range of buoyancy numbers.